class Solution {
public:
    int numDecodings(string s) {
        int n = s.size();
        vector<int> f(n + 1);
        f[0] = 1;  
        for(int i = 1; i <= n; i++){
            if(s[i - 1] != '0') f[i] = f[i - 1];       
            if(i >= 2){
                int t = (s[i - 2] - '0') * 10 + s[i - 1] - '0';
                if(t >= 10 && t <= 26) f[i] += f[i - 2]; 
            }
        }
        return f[n];
    }
};


